1. Field of the Invention
The present invention relates to a method of producing a foam-insulated electric wire having an insulating layer of a polyolefin foam with a high cell content and high strength.
2. Description of the Prior Art
For example, as a cable for interconnecting computers, a foam-insulated electric wire has widely been in use, wherein a conductor is coated with a foam insulating layer of, for example, a polyolefin resin, thereby increasing a velocity of signal propagation (vop) as much as possible.
An example of this method of producing a foam-insulated wire will now be described with reference to FIG. 1.
A screw extruder 1 has a rear end portion 1a provided with a resin inlet 2, and an extrusion port 1b provided with a crosshead portion 3 having a die. A polyolefin resin such as polyethylene is supplied into the resin inlet 2 of the screw extruder 1. The die is held within the crosshead portion 3 so that the axis of the die intersects at right angles with the axis of the screw extruder. A conductor 4 is continuously guided into the die in the direction of the arrow.
The temperature of the screw extruder 1 is controlled at a predetermined value. The resin supplied into the resin inlet 2 is conveyed to the extrusion port 1b, as being melted and kneaded by the screw of the extruder 1. At this time, a predetermined amount of a foaming agent (also referred to as a "blowing agent") is supplied into a foaming agent inlet formed between the resin inlet 2 and the extrusion port 1b.
The supplied resin with some additives and supplied foaming agent are kneaded in the extruder 1 into a molten mixture which is about to foam. The mixture is supplied from the extrusion port 1b to the crosshead portion 3. Then, the mixture is extruded from the die held in the crosshead portion 3. The molten mixture coats the periphery of the conductor 4, which continuously runs through the die in the direction of arrow, and is foamed into a foam-insulating layer.
In this way, the conductor 4, which runs continuously through inside the die, is coated with the foam-insulating layer by the foam extrusion process, and an object product or a foam-insulated electric wire 6 is obtained.
Foaming agents in use for the foam extrusion coating process are chlorofluorocarbon (known as "Flon"), nitrogen, carbon dioxide, propane, or butane, which are gasses at a room temperature and under atmospheric pressure and does not react with polyolefin resin, or an organic solvent having a low boiling point, such as n-pentane, n-hexane, methanol or ethanol. In particular, Flon has widely been used because of the handiness and effectiveness as a foaming agent.
Of the above foaming agents, a gas agent is supplied in a predetermined amount into a foaming agent inlet 5 by using a gas flowmeter or a gas pressure regulator. An organic solvent having a low boiling point is also supplied into the inlet 5 by using a feeding pump or the like. In the case of a gas which is a gas at a room temperature and under atmospheric pressure, like some Flon, and has a low vapor pressure, such a gas is once pressurized under a relatively low pressure to be liquefied and then supplied.
Flon, which has been dominantly employed as a foaming agent, depletes the ozone layer. It is thus planned that the use of specified Flon having a considerable ozone depletion potential is to be completely phased out by the year of 2000 and the use of other kinds of Flon is to be totally phased out in the not too distant future.
Against this background, in the field of production of foamed polyolefin, the development of a pollution-free foaming agent substituted for Flon is strongly required.
For foam-insulated electric wires used in a cable for high-speed signal transmission, the foaming degree of the foam insulating layer coated on a conductor has been increased, and a diameter of these wires has been reduced.
However, the more subsisting cells of a foam insulating layer coated on a conductor increase and the thinner the thickness of a foam insulating layer becomes, the more the toughness of the insulating layer is sacrificed. Thus, the foam insulating layer may be squeezed and the conductor may be exposed owing to the force arising when the terminal of the foam-insulated electric wire is processed during wiring work, or the insulating layer may be squeezed by external pressure, causing a disadvantage in that electrical characteristics of the foam-insulated electric wire, such as velocity of signal propagation (vop), characteristic impedance, and capacitance, tend to shift.
In addition, the thinner the thickness of a foam insulating layer, the smaller the quantity of a foaming agent supplied in the foaming process. In this case, it is necessary to exactly control the quantity of a supplied foaming agent. If the quantity of supplied foaming agent were uneven, the thickness of the foam insulating layer and the electrical characteristics of the foam-insulated electric wire would also become unstable.
In the meantime, it is very difficult to exactly supply a small amount of the aforementioned gas by finely controlling the quantity of the gas by means of a flowmeter or the like. If the gas is liquefied, it becomes possible to exactly supply a small amount of the gas, but the liquefying process requires very high pressure or very low temperature. This is not industrially advantageous from the viewpoint of safety and production cost.
It appears that, as a foaming agent substitutable for Flon, the aforementioned organic solvent having a boiling point is suitable, and it can exactly be supplied in a minute quantity.
However, in the case where a foam with a high cell content is produced by using an organic solvent having a lower boiling point, if a cell content increases, the toughness of the foam decreases accordingly and the deformation factor at normal temperature increases.